Direct measurement of vessel diameter and intravascular pressure will be used to localize the primary size of local vascular control of intestinal blood flow during absorption of glucose. In addition, a direct optical-electronic technique will be used to measure capillary blood flow in the muscular, submucosal and mucosal layers of the intestine during rest and absorption periods. These data will be used to determine the relative involvement of the microvasculature in each layer of the intestine during absorption by the mucosa. The distribution of tissue and perivascular partial pressure of oxygen (PO2) in each section of the intestinal microvasculature will also be measured. These data will be used to define how the mucosal microvasculature affects tissue PO2 at rest and the changes in tissue and perivascular PO2 throughout the intestine which occur during absorption. The purpose of these measurements is to determine if tissue PO2 is regulated or simply determined by the blood flow and tissue metabolism at the time of measurement. Additional experiments will be conducted to determine the dependence of the hyperemia during absorption on the tissue and environmental PO2. The increase in blood flow above control will be measured during a constant rate of glucose absorption. These measurements will be made at environmental PO2's which are normal and above and below normal. If the hyperemia during absorption is independent of the tissue or environmental PO2, it is likely that factors related to metabolites are the cause of the hyperemia rather than oxygen.